The researchers say the amphibians ate by biting down with a big chomp on their meal.

"They don't capture prey like fish. They don't suck it into their mouths. Instead, they captured prey more like what you'd think of a crocodile doing or a lizard," says Markey.

The study sheds light on a key evolutionary change during a crucial time in the history of life, as vertebrates were first leaving the water and colonising dry land.

This transition set the stage for the appearance of the later major groups of land vertebrates like reptiles, birds and mammals.

The researchers first examined the skull structure of the living African freshwater bichir fish, a primitive species with some similarities to fish alive 300 million years ago.

They found that the shape of certain features on the skull roof indicated whether a fish caught its prey by sucking it into the mouth, much like a goldfish, or by biting on it directly like a crocodile.

They then examined similar features on the fossilised skulls of three extinct animals.

Acanthostega, which likely lived almost exclusively in the water and was one of the first tetrapods to appear in the fossil record, caught prey more like later land animals, with a big bite, the researchers say.

"This was unexpected because even though Acanthostega has legs and is an early tetrapod, it still has internal gills (like a fish). So people had thought that this meant that it would spend most of its time in the water," Markey says.

"You would expect if that's the case then it might capture its prey more like a fish. But it didn't."

The fish Eusthenopteron, which also lived 370 million years ago and had fins that may have enabled it to crawl onto land for short spells, caught its prey by sucking it into its mouth, they found.

Fish like Eusthenopteron are seen as precursors to amphibians, which live both in the water and on land.

Not surprisingly, the researchers found the Phonerpeton, a more advanced amphibian that had robust limbs and was fully land-dwelling dating from about 275 million years ago, also chomped its prey.